The invention relates to a direct method for removing mercury from wastestreams by regenerative adsorption.
It is desirable to remove mercury from the liquid discharge from various industrial processes, particularly to meet environmental concerns. Mercury may be present, for example, in the effluents from chlor-alkali plants, in water produced from gas and oil fields and in the discharge from power plants and certain petroleum refining processing units.
The mercury concentration permissible in drinking water under Environmental Protection Agency guidelines is about two parts per billion. The solubility of Hg in water depends on the purity of the water. The solubility of pure mercury in pure drinking water is about 20 to 30 ppb, but the solubility increases with alkalinity and mercury solubility can be as high as 54 ppb in aqueous 5% potassium hydroxide solution, increasing to 135 ppb in aqueous 10% potassium hydroxide. The mercury content in water from a reflux condenser of a crude oil tower used in petroleum processing was found to be greater than 100 ppb.
The mercury in wastewater can contaminate the ground through adsorption. In addition, mercury in the ground or in waterways converts quite readily to volatile, toxic alkyl mercury. Because of its high vapor pressure, the alkyl mercury can escape into the air resulting in air pollution.
An indirect method for removing mercury from wastewater has been proposed in U.S. patent application Ser. No. 297,157 filed Jan. 17, 1989, now U.S. Pat. No. 4,962,276. In this indirect method, mercury is stripped from the wastewater using a stripper gas such as N.sub.2 or air. The mercury-containing gas is then treated over an adsorbent. For 90% Hg removal, 1000 cc of N.sub.2 per cc of wastewater may be required so that the cost of gas compression becomes significant. A direct method for removing mercury from wastewater by causing the mercury to react directly with an adsorbent would eliminate this cost.
Various processes have been disclosed for removing mercury from liquids. U.S. Pat. Nos. 2,860,952, 3,674,428, 3,749,761, 3,790,370, 3,847,598, 4,053,401, 4,147,626 and 4,474,896 describe methods for reducing the mercury content of aqueous solutions. In U.S. Pat. Nos. 2,860,952, 3,674,428, 3,749,761, 3,790,370, 4,053,401 and 4,147,626, a sulfur-containing compound is added to the effluent to be treated resulting in an insoluble sulfide product which is then removed. In U.S. Pat. No. 3,847,598, a stream of inert gas is passed through the aqueous solution in the presence of a reducing agent which reduces inorganic mercury compounds to elemental mercury.
Various adsorbents have been described for removing mercury from hydrocarbon-containing liquid or gaseous streams. U.S. Pat. No. 4,474,896 describes a mercury adsorbent having a polysulfide and a metal cation on a support. French Patent No. 75/15579 discloses a pelletized regenerable adsorber mass containing a support of silica and/or alumina compounds with crystallites of gold, silver, or copper compounds which have been impregnated onto the support. The adsorbent is used to eliminate mercury from natural gas containing up to 50 Mg Hg/m.sup.3 of gas. The problems confronted in removing mercury from streams which are primarily hydrocarbon differ from the problems which exist in removing mercury from wastestreams which are primarily aqueous. The concentration of mercury in wastestreams from industrial processes can be much higher than the mercury content of natural gas. Mercury in wastewaters from chlor-alkali plants is particularly high. In addition, hydrocarbon contaminants and inorganic compounds present in the wastewater, for example, from petroleum processing can compete with mercury for adsorption to reduce the effective removal of mercury and reduce the capacity of the adsorbent. An adsorbent which is selective for Hg over contaminants in the wastewater and a process for removing Hg selectively from wastewater are clearly needed.
The use of sodium borosulfide has been suggested for removing Hg from wastewaters by Cook, et al. Pollution Eng. 36-38, December 1981. Sodium hydrosulfide and sodium polysulfides have been used for Hg removal by Findlay, et al. Environmental Science & Technology 15 (No. 11) 1298-1390 (1981). However, wastewaters often contain other compounds which also react with the treating agent resulting in high chemical consumption and generation of new waste for disposal.
It is therefore an object of the invention to provide a direct adsorption method for removing mercury from wastestreams which contain mercury.